In 1998 two competing teams of astronomers set out to measure how fast the expansion of the universe was slowing down. Gravity pulls; the universe is full of matter; therefore the expansion that began with the Big Bang should be gradually braking. The only question was by how much.
Both teams got the wrong sign. The expansion was not slowing. It was accelerating.
This was one of the most unwelcome results in the history of physics, and it held up. Both teams checked it independently and reached the same conclusion, and the 2011 Nobel Prize in Physics went to Saul Perlmutter, Brian Schmidt and Adam Riess for the discovery. Whatever is causing it was named dark energy — a placeholder for something nobody understands.
How you measure the expansion of the universe
The method depends on a particular kind of exploding star: a type Ia supernova.
These occur in binary systems where a white dwarf pulls matter from a companion star. When the white dwarf reaches a critical mass — around 1.4 times the mass of the Sun — it detonates. Because the trigger point is always roughly the same mass, the explosion always releases roughly the same amount of light.
That makes type Ia supernovae standard candles. If you know how bright something truly is, and you measure how bright it appears, you can calculate its distance. Meanwhile the redshift of its light tells you how much the universe has expanded since that light was emitted.
Combine distance with redshift across many supernovae at different distances and you can reconstruct the expansion history of the universe.
What the data actually showed
The distant supernovae were fainter than they should have been. Fainter means further away than a decelerating universe would place them.
Reading the history backwards, the picture is this: for roughly the first 8 billion years, matter dominated and the expansion did slow down, as expected. Then, around 5 to 6 billion years ago, something changed. As matter thinned out with expansion, the density of matter dropped below the density of whatever dark energy is — and the universe began to accelerate.
We happen to live in the era after that crossover. Had astronomers looked 6 billion years earlier, they would have found the deceleration they were expecting.
The energy budget of the universe
The proportions, from measurements of the cosmic microwave background and large-scale structure:
- Dark energy: about 68 per cent
- Dark matter: about 27 per cent
- Ordinary matter — atoms, stars, planets, gas, dust, you: about 5 per cent
Everything ever observed through a telescope, every element in the periodic table, every living thing, accounts for roughly a twentieth of the contents of the universe. The rest is two separate things we cannot see, and they are not the same thing. Dark matter has gravity and pulls structures together. Dark energy pushes space apart. They are near-opposites that happen to share an adjective.
The candidate explanations
The cosmological constant. Einstein added a term to his equations of general relativity in 1917 — lambda — to allow a static universe, then dropped it when expansion was discovered. The accelerating universe brings it back: lambda behaves like an energy density built into empty space itself, constant everywhere and always. This is the simplest explanation and it fits the data well.
Vacuum energy. Quantum field theory predicts that empty space is not empty; it seethes with fields at their lowest energy state, which carries energy. This looks like an excellent candidate for the cosmological constant, except for one problem. When you calculate how much vacuum energy there should be, the answer exceeds the observed value by something between 50 and 120 orders of magnitude, depending on how you cut off the calculation. This is the vacuum catastrophe, widely described as the worst theoretical prediction in the history of physics.
Quintessence. Instead of a constant, a dynamic field that changes over cosmic time. This would mean dark energy was different in the past and will be different in the future. It is harder to test but not ruled out.
Modified gravity. Perhaps there is no dark energy and general relativity needs correcting on the largest scales. Various models exist, but general relativity has survived every precision test thrown at it, including gravitational wave observations, which makes this a demanding route.
The Hubble tension, and hints of change
Two independent methods of measuring the current expansion rate disagree. Measurements based on the cosmic microwave background give a value around 67 to 68 kilometres per second per megaparsec. Measurements based on nearby supernovae and other distance ladders give around 73. The error bars do not overlap comfortably, and the gap has grown as measurements have improved rather than shrinking as measurement errors usually do.
Nobody knows whether this is an unrecognised systematic error or a sign that the standard cosmological model is incomplete.
Separately, in 2024 and 2025 the Dark Energy Spectroscopic Instrument released results hinting that dark energy may not be perfectly constant after all — that its density may have weakened slightly over cosmic time. The signal is not yet at the certainty level physicists require for a claim of discovery, and it may evaporate. But if it holds, the cosmological constant would be wrong, and quintessence-style models would move to the front.
How the universe ends, depending on the answer
The nature of dark energy determines the fate of everything.
- If dark energy stays constant, expansion continues to accelerate. Distant galaxies eventually recede faster than light can cross the gap, and one by one they leave our observable universe. Star formation ends as gas is exhausted, stars burn out, and the universe drifts toward a cold, dark, near-empty state. This is heat death, and it is the default expectation.
- If dark energy strengthens over time, expansion accelerates without limit until it overwhelms the forces holding galaxies, then stars, then atoms. This is the Big Rip.
- If dark energy weakens and reverses, gravity eventually wins and the universe recollapses. This is the Big Crunch, now considered unlikely on current data.
Astronomers in the very distant future would have a much harder time working any of this out than we do. Once other galaxies have receded beyond view, the evidence that the universe expands at all becomes almost impossible to gather. We are living in a relatively brief window when the history is still readable.
Summary
Dark energy is the name for whatever is causing the expansion of the universe to accelerate. It makes up roughly 68 per cent of the total energy content of the cosmos, it was discovered by two teams who were looking for the opposite result, and after nearly three decades of work nobody knows what it is.
The leading explanation, a cosmological constant built into empty space, is elegant, fits the data, and disagrees with quantum field theory's prediction of its size by a factor so large that it is usually written as a power of ten rather than a number.
That is an unusual state of affairs to be honest about: the dominant component of the universe is something we detected only through its effect on the light of exploding stars, and our best theory of it is off by up to 120 orders of magnitude.
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